Conventional methods, such as those disclosed in DE 195 11 548 A1, DE 198 01 815 A1 or DE 199 53 601 A1, with the last-mentioned document also disclosing evaluation of a catalytic converter as regards ageing by evaluating the air mass value of the combustion air that is sucked in by the internal combustion engine.
In the case of internal combustion engines, emitted exhaust gases can be given an aftertreatment in the exhaust gas duct by using a three-way catalytic converter that oxidizes or reduces harmful substances of the exhaust gas to innocuous compounds. However, it is also known that such internal combustion engines equipped with a three-way catalytic converter for achieving a high degree of efficiency must be supplied with an average stoichiometric air/fuel mixture; in such a lambda regulation, the oxygen contents of the exhaust gas is measured by means of so-called lambda probes and the air/fuel mixture is regulated to an average value close to lambda=1 because three-way catalytic converters only function in a tight range around lambda 1 as requested. This range is also designated as the catalytic converter window.
In order to increase the degree of efficiency of a three-way catalytic converter, the air/fuel mixture is designed in such a way that in the forced activation acting on the lambda regulation as anticipatory control around the stoichiometric set value, default values are set alternately with an over-stoichiometric and under-stoichiometric mixture in cycles. Because of the forced activation, the default value for the lambda value in rich phases is lower than the stoichiometric set value and in lean phases is greater than the set value.
Alternately, supplying oxygen to and extracting oxygen from the three-way catalytic converter results in suitable oxygen ratios for the oxidation and reduction phases.
Because the reducing or oxidizing effect of a three-way catalytic converter decreases tremendously in the case of values set below or above a set value for the stoichiometric mixture, care must be taken that in the forced activation within the average time, an air/fuel mixture is always used in the catalytic converter window.
Therefore, in the prior art in lean and rich phases of the forced activation, a default value deviating by the same amount from the stoichiometric set value is set in each case and the phases are equal in length. Lambda deviations from the default value possibly determined by interference are balanced out by a lambda regulator.